1. Field of the Invention
The present invention relates to an electronic balance, and more particularly to an electronic balance utilizing as a balancing force an electromagnetic force which is automatically controlled so as to be balanced against a weight to be measured.
2. Prior Art
In general, known electronic balances have a force equilibrating unit comprising an electromagnetic coil and a magnetic circuit to provide a static magnetic field, the electromagnetic coil being mechanically connected with a weight-receiving tray and supported movably in a vertical direction in said static magnetic field. The electromagnetic coil, supplied with an electric current, produces an electromagnetic force to oppose a weight placed on the weight-receiving tray. The weight value is given by the current which makes the electromagnetic force just balanced against the weight to be measured.
In relation to this type of balances the present patent applicant has already filed an electronic balance in which the electromagnetic coil is supplied with an "alternating" pulse current, namely, a pulse current whose pulse polarity is alternating. The frequency of the pulse is chosen sufficiently high so that the balance may be prevented from possible mechanical vibrations. The electromagnetic force opposing the weight is regulated by automatically controlling the duty factor of the pulse current, with the amplitude and frequency kept constant. The average current, which determines the electromagnetic force, depends on the duty factor of the pulse. The use of an "alternating" pulse current is to keep the Joule heat generation by the coil always constant irrespective of the average value of the pulse current. There are also disclosed some other kinds of electronic balance in which a "non-alternating" pulse current is used. In any way, the most important problem involved in an electronic balance in which a pulse current is used is to measure the duty factor of the pulse so precisely as to comply with a resolving power required for the balance. The duty factor of a pulse current is, in general, measured with a clock signal whose frequency is sufficiently higher than that of the pulse. In principle, therefore, the resolving power can be increased to any degree by increasing the frequency of the clock. In practice, however, there are some technological and economic difficulties and disadvantages in increasing the clock frequency on a large scale. In case of a balance using a 1 kHz-pulse current a 1 MHz-clock gives a resolving power of the order of only 10.sup.-3. If, as is often the case with a precision balance, a resolving power of the order of 10.sup.-6 is needed, a 1 GHz-clock must be used. The use of such a high frequency clock in a balance is apparently unpractical in both technological and economic aspects. Although there have been proposed, of course, some contrivances for measuring the duty factor, they also have many disadvantages, particularly in relation to the requirements for the measuring speed and precision.